The objective of the invention described in said patent is to provide a simple and easy to manufacture shearing interferometer instrument preferably portable, for measuring the degree of collimation of a coherent light source (e.g. a laser) used to establish alignment(s) in construction projects, machinery and other uses where a laser beam is directed over substantial distances. Such uses ordinarily result in some spreading of the beam at long distances (e.g. 40 to 100 meters or more) from the beam source. The present invention utilizes the shearing interferometer for collimation measurement and also improves the sensitivity of At the technique at the least by a factor of two.
In accordance with the patented invention, a simple image shearing (shifting) plate interferometer is comprised of a transparent double-wedge optical element (plate) located at an angle (preferably 45 degrees) from the incoming beam to be measured. The basic principle of operation involves directing a coherent beam of light, to be measured for collimation input, along a determined axis incident at 45 degrees onto the double-wedge element reflecting/refracting sides. Some of the light reflects off a first surface of each of the wedge plates and interferes with light reflected off the second or other surfaces of the wedge plates. This first and second reflected light is directed to a view screen, where there appears resultant side-by-side images of the interference patterns from each wedge plate of the optical element.
The comparative fringe patterns do not rotate with respect to each other, but the number of fringes increases on one side with respect to the other, depending upon whether a diverging or converging beam is directed onto the double-wedge plate element. When the fringe spacing on both sides of the viewing screen appears identical, this indicates a collimated input beam.
In a preferred embodiment of such interferometer the two wedge plates have non-parallel surfaces arranged to receive a portion (e.g. one-half) of the input light beam, and project the side-by-side interference patterns directly onto a viewing screen. The instrument incorporates a rectangular, preferably cube shaped, housing having an input window in one side of the housing for admission of the beam being investigated, and a viewing window in another side located at right angles to the input window. A viewing screen (such as a diffusing plate) is fitted into or ail supported in the viewing window. The optical element is supported in and across the housing extending at forty-five degrees to the input window, from a corner adjacent the viewing window to an opposite corner. This is simple and economical arrangement, retaining a high degree of accuracy and ease of comparison of the two images, and being capable of holding in one hand of the user.
With this shearing interferometer it is simple to determine when a beam of light is collimated. When the sheared wavefronts are made to appear equivalent, then the beam is collimated. Actual measurement of the amount of convergence or divergence of the beam is possible, but is difficult and results in an indirect measurement. One would have to compare the number of fringes over a given distance on one half of the view screen with the number of fringes within the same distance on the other half of the view screen. The number of fringes would then become a variable in an equation which also requires wavelength, wedge angle, etc.